Carbonless copy paper is disclosed in expired U.S. Pat. No. 2,711,375 issued to Robert W. Sandberg. This patent describes pressure-rupturable hydrophilic colloid material enclosing discrete liquid inclusions of an oily marking substance. The protective material used in this early coating was interlaced fibers of cellulose. Paper coating starch binder is disclosed as an optional ingredient. However, the cellulose fibers were considered as the "anti-smudge" means in this patent (column 2, lines 1-3 and lines 53-56).
U.S. Pat. No. 3,574,133 issued Apr. 6, 1971 to Bayless et al. discloses microencapsulated coatings, and makes reference to many related applications. The reference mentions that the microcapsule size ordinarily ranges from about 2-1,000 microns. This patent is primarily directed to the manufacture of the minute capsules, however, and does not add anything to the solution of the smudging problem. The capsule size and function would indicate that there was a problem in handling paper substrates coated with these capsules, too.
Another related patent application was filed by Robert Bayless and Donald Emrick on the same date as the above reference. U.S. Pat. No. 3,565,818 attempts to solve the premature rupture problem by chemically treating the capsule walls with certain transition metal salts to harden them and increase their rigidity.
Netherlands patent application No. 7,005,045 opened for public inspection on Oct. 12, 1970, describes the smudging problem with carbonless duplicating paper coatings in much more detail, and discloses the use of a "stilt" material which is distributed uniformly in the coating to protect the capsules from premature rupture while the coated paper is being handled. The particulate "stilt material" is uniformly distributed through the coating, and it is desirable that the stilt particles be a little larger than the microcapsules to give the best protection. The known stilt materials described in this Dutch patent application are: fine glass beads, short cellulose fibers, and starch grains. The starch grains are considered effective and inexpensive stilt materials, but only starch grains of the correct size are considered useful to protect the microcapsules. As a rule of thumb, this Dutch application suggests the starch particles should be 1.2 times the average size of the microcapsules. The Dutch reference lists the following starch granules as possible "stilt" materials:
______________________________________ Average Particle Type of Starch Size in Microns FS TI ______________________________________ arrowroot 25-50 93 50 potato 15-100 88 48 sago 20-60 84 49 wheat 2-35 78 50 tapioca 5-35 78 54 maize 5-25 73 51 rice 3-8 64 52 ______________________________________
Even though the above list is extensive, it does not include legume starches such as yellow field pea starch and faba bean starch. The above tabular summary includes values determined experimentally for the friction staining (FS) by coated papers containing the above stilt materials interspersed with ink containing microcapsules in the coatings. The tests are performed in a manner to simulate handling pressure applied to the coated paper samples, a value of 85 or more is considered desirable for the friction stain test, but it must be balanced by the requirement that a reasonable level of pressure should cause the capsules to rupture and form a copy image. The test devised to measure this value is referred to as the typewriter intensity (TI) test, and it is also described in the subject Dutch patent application No. 7,005,045. A TI value of 55 or less is considered good. It can be seen from the above tabular comparison, only arrowroot and potato starches exhibited the desired friction stain (FS) value in equilibrium with the desired typewriter intensity (TI) value. Sago starch was probably close enough to be considered, but as a practical matter, all three of the above starches are too costly or too scarce to be considered useful as a "stilt" material in microcapsule coatings. (See also Canadian Pat. No. 879,038). Potato starch granules are too large to be used.
U.S. Pat. No. 3,951,948 issued Apr. 20, 1976 having a common assignee herewith discloses a method and apparatus for physically separating a large granule starch stilt material from a starch having a large granule portion. The unwanted, interfering small granules are separated from the large granule portion by a means therein disclosed which has proven effective, but it adds processing cost to the stilt material.